The present invention relates in general to audio systems which prevent distortion from amplifier clipping by employing voltage limiting, and more specifically to automotive audio systems using at least two voltage limiting feedback loops for different frequency ranges in the audio spectrum.
Typical audio reproduction systems include a variable-gain amplification stage followed by a fixed-gain power amplifier which feeds an output transducer such as a speaker. A control voltage provided to the variable gain stage controls the output volume heard by a listener. The variable gain stage usually includes a tone control for varying the relative amplification of bass and/or treble frequency ranges.
An important objective in designing an audio system is to provide minimum distortion in signal reproduction. However, there is always some distortion, especially at high sound levels. As the magnitude of the signal provided from the variable gain amplifier stage to the power amplifier increases above a certain level, the power amplifier becomes overdriven. This situation occurs when the input signal to the power amplifier multiplied by the fixed gain of the power amplifier approaches the supply voltage level provided to the power amplifier. As a result, the power amplifier becomes saturated and signal peaks of the audio signal are distorted by clipping.
The problem of power amplifier clipping is more severe in automotive audio systems. Less voltage headroom (i.e., safety margin) is available to the power amplifier since the automobile is limited to a 12-volt electrical supply. Although a DC/DC converter can be used to obtain a higher DC voltage, such converters are relatively expensive. Also, bass boost is needed in the automotive environment to mask low frequency road and engine noise, making clipping more likely in the bass range of the audio signal.
It is known to employ voltage limiting or compression to the input of an amplifier to prevent clipping. In Evans et al, U.S. Pat. No. 4,048,573, a high fidelity audio amplifier compares the amplifier input to the amplifier feedback signal. The input signal is attenuated when any excessive signal in either the input or the feedback is detected. The resulting amplifier is wideband limited, i.e., even though the large amplitude signals causing limiting may be substantially only within a restricted range of frequencies, all frequencies are attenuated which results in a "breathing" effect. Thus, a loud bass signal can cause the reduction in volume of a quieter treble signal.
In Mestorovic, U.S. Pat. No. 4,233,566, the power amplifier output signal is compared to a predetermined reference. When the output signal exceeds the reference, a voltage controlled attenuator reduces the input signal to the power amplifier. This system is likewise subject to the breathing effects caused by its wideband approach.
Sondermeyer, U.S. Pat. No. 4,318,053, discloses an amplifier which reduces the amplifier gain when clipping reaches a certain threshold. A threshold detector charges a capacitor which may then be discharged into a variable gain amplifier according to the percent of clipping of the output signal of the amplifier. This system has the disadvantages that limiting is performed over the entire frequency range of the amplifier even upon the occurrence of a large amplitude signal of narrow frequencies.
The paper Noble et al, A Dual-Band Audio Limiter, Journal of the Audio Engineering Society, Vol. 17, No. 6, December 1969, pages 678-684, describes a limiting system in which the audio signal is split into separate paths for bass frequencies and higher frequencies. Gain in each path is reduced upon the occurrence of a peak level, the reduced gain being maintained for the recovery time of the limiter. Noble et al point out that recovery time should be short in order to minimize dynamic distortion. However, harmonic distortion and intermodulation distortion require a longer release time to minimize the distortion. Noble et al deals with these conflicting results by providing different release times in each path. Thus, distortion is reduced for bass frequencies by providing a longer release time, while distortion at higher frequencies is minimized by a short release time. However, the threshold for initiating limiting is the same in each band of frequencies.
The dual-band limiter in Noble et al has the problem that a phase splitter and band pass filters are required to separate the audio signal into separate bands and that the separate bands must be mixed together in the output. These elements add their own distortion to the signal and expense to the amplifier.